1. Field of the Invention
The present invention relates to an optical modulator that modulates light with light to generate an m-ary optical signal for use in, for example, long-haul high-capacity fiber-optic communication, where m is an integer greater than two.
2. Description of the Related Art
Due to the pervasive spread of the Internet, the need for long-haul large-capacity optical fiber communications has been increasing. Communication capacity is being enlarged in two ways: by using wavelength division multiplexing (WDM) to increase the number of simultaneously transmittable channels, and by increasing the transmission rate in each channel.
M-ary modulation, which is already in use in mobile radio communication systems, is now attracting attention as one possible means for increasing the capacity and range of optical communication. Many researchers are currently studying its possible application to optical fiber communication systems.
The two major optical communication systems that have been put into practice or are under study are amplitude shift keying (ASK) or on-off keying (OOK) modulation, in which the signal allocated to each time slot has either a weak (‘0’) or strong (‘1’) intensity, and binary phase shift keying (BPSK) modulation, in which the signal allocated to each time slot has a phase shift of either 0 or π radians. In both of these modulation systems, only one bit (two possible values) can be transmitted at once.
A typical m-ary modulation scheme is quadrature phase shift keying (QPSK). In QPSK, the phase of the signal in a single time slot may shifted by 0, π/2, π, or 3π/2 radians, enabling the transmission of two bits (four possible values) at once.
If used in optical fiber communications, QPSK modulation would allow twice as much data to be transmitted in the same frequency band as by OOK or BPSK modulation, resulting in increased communication capacity and improved spectral utilization efficiency. Conversely, since QPSK uses only half as much bandwidth as OOK or BPSK modulation to transmit the same amount of data, when QPSK is used in a WDM system, the wavelength channel spacing can be reduced, increasing the communication capacity and again improving the spectrum utilization efficiency. The reduced bandwidth would also make the transmitted signal less vulnerable to waveform distortion due to group velocity dispersion in the optical fiber, so another advantage of QPSK would be an increased communication range.
The optical QPSK modulators now under study are typically electro-optical (E/O) systems that convert electrically modulated signals to optically modulated signals. An exemplary system of this type is described by Kawanishi et al. in ‘80 Gb/s DQPSK modulator’, Technical Digest of OFC 2007, OWH5, 2007.
Gb/s is an abbreviation for gigabits per second. The abbreviations Gbps and Gbits/s are also used.
The system described by Kawanishi et al. employs Mach-Zehnder (MZ) interferometric lithium niobate (LiNbO3) modulators, which exploit the Pockels effect in an LiNbO3 crystal. Two such modulators (MZA and MZB) are used to generate a pair of 40-Gb/s BPSK signals, which are then combined in an optical coupler (MZC) to generate an 80-Gb/s QPSK signal.
In this and other known electro-optical QPSK modulators, the bit rate of the QPSK signal is limited by the operating speed of the component E/O modulators. In order to obtain faster bit rates, it is necessary to increase the operating speed of the electronic devices that generate the electrically modulated signals as well as the electro-optic conversion speed of the E/O modulators themselves. The state of the art in commercially available devices is currently about 50 Gbps, limiting the QPSK signal to about 100 Gbps.
To generate QPSK signals beyond the limits of electronic devices and E/O optical modulators, it would be preferable to use an all-optical modulator in which the signal light is modulated by an optical modulating signal or control signal.
A preferred optical modulation method uses the optical Kerr effect in an optical fiber. The optical Kerr effect occurs when the refractive indexes of a fiber vary due to propagation of light with high intensity in the fiber. The response speed of the optical Kerr effect is on the order of a few femtoseconds.
An exemplary method of fabricating an ultra high-speed optical modulator or switch by utilizing the fiber-optic Kerr effect has been described by Morioka et al. in ‘Ultrafast optical multi/demultiplexer utilising optical Kerr effect in polarisation-maintaining single-mode fibres’, Electronic Letters, Vol. 23, No. 9, pp. 453-454, 1987. This type of optical fiber has two axes, referred to as the slow axis and fast axis, in a plane orthogonal to the longitudinal axis of the fiber. Linearly polarized light propagating through the fiber experiences different effective indexes of refraction depending on whether the light is polarized parallel to the fast axis or the slow axis.
The Kerr medium used by Morioka et al. includes two polarization-maintaining optical fibers spliced end-to-end with mutually orthogonal slow axes so that the birefringence of the two fibers cancels out. In the experiment described by Morioka et al., linearly polarized OOK-modulated control light pulses and unmodulated probe light pulses were coupled into this medium, respectively polarized parallel to and at a 45° angle to the fiber axes. A pulse of probe light propagating through the medium together with a pulse of control light had its polarization plane rotated by the Kerr effect, which produced a phase difference φ between the probe light components polarized parallel to and orthogonal to the control light. The intensity of the control light could be adjusted to create a phase shift φ of π radians and thus a polarization rotation of 90°. When no control light pulse was present, there was no net phase shift and the polarization plane of the probe light pulse was not rotated.
This experiment demonstrates that the fiber-optic Kerr effect can transform an OOK or ASK modulation pattern into a phase modulation pattern and suggests that the fiber-optic Kerr effect could be used to realize an all-optical BPSK modulator operating at a bit rate of at least several hundred gigabits per second. It is easy to infer that a QPSK optical signal could be generated by combining two BPSK signals generated in this way in an optical coupler such as coupler MZC described by Kawanishi et al.
Generating an optical QPSK signal by combining two optical BPSK signals, however, requires precise control of the phase relationship between the two optical BPSK signals. In the typical case in which the two optical BPSK signals are modulated with phases of 0 and π, for example, an ideal optical QPSK signal is not obtained unless the phase difference between them is precisely π/2.
The phase of the individual optical BPSK signals is not determined solely by the electrical modulating signal used by Kawanishi et al. or the optical control signal used by Morioka et al.; the phase is also shifted by the optical lengths of the individual paths taken by the optical signals.
If the optical modulation scheme proposed by Morioka et al. is used, an optical fiber with a length of from several tens of meters to several kilometers is required to obtain an adequate optical phase modulation effect from control light of a practical intensity. This length is millions or billions of times the wavelength of the optical signal. Precise control of the relative phases of two optical signals propagating through fibers of this length would be extremely difficult; the necessary phase control equipment would have to respond at high speed with high precision to measured phase changes, and would also have to compensate for phase drift due to temperature changes and other environmental factors. Such a phase control system would be prohibitively complex and expensive.
Thus while it is easy to conceive of an optical QPSK modulator using an optical coupler such as coupler MZC in Kawanishi et al. to combine two optical BPSK signals generated by the optical modulation technique described by Morioka et al., a practical optical QPSK modulator of this type would be extremely difficult to build and would require complex and very costly optical phase control apparatus.